The hysteretic energy loss of a rubber vulcanizate is commonly associated with the number of free polymer chain ends. For example a polymer with a high molecular weight exhibits a decreased weight content of the end groups and has a reduced hysteresis loss but also results in reduced processability of the rubber compounds. The functionalization of chain ends with polar groups suitable to interact with filler leads to a reduced amount of free chain ends and usually reduces hysteresis. Nevertheless, a high amount of chain end-filler interactions leads to a remarkably reduced workability of the compound.
Elastomeric polymers obtained by anionic polymerization processes mainly contain linear polymer chains. Highly linear elastomeric polymer chains show high solution viscosities and cold flow properties. To overcome these drawbacks, coupling processes have been applied to generate at least partially branching of the polymer chains. Commonly used coupling agents are divinylbenzene, halide or alkoxide compounds of tin or silicon. However, branched or star shaped polymers which result from such coupling reactions often exhibit increased hysteresis loss or decreased abrasion resistance.
WO 2007/047943 describes the use of a silane sulfide omega chain end modifier to produce a chain end-modified elastomeric polymer which is used as component in a vulcanized elastomeric polymer composition or a tire tread.
According to WO 2007/047943, a silane sulfide compound is reacted with anionically-initiated living polymers to produce “chain end-modified” polymers, which are subsequently blended with fillers, vulcanizing agents, accelerators or oil extenders to produce a vulcanized elastomeric polymer composition having low hysteresis loss.
The vulcanized elastomeric polymer compositions are described as exhibiting lower tan δ values at 60° C., particularly as compared to compounds based on corresponding non-modified polymers, without negatively affecting tan δ values at 0° C. and processing characteristics, such as compound Mooney values. Lower values of tan δ at 60° C. correspond to a lower rolling resistance, whereas a higher tan δ at 0° C. corresponds to an improved wet grip of a tire. Exemplary cured polymer formulations have been shown to result in reduced tan δ at 60° C. and heat build-up values but equivalent tan δ values at 0° C. They are described as being useful in preparing tire treads having lower rolling resistance, while maintaining good wet grip properties. Although cured rubber hysteresis properties can be improved significantly through application of the technology described in WO 2007/047943, the impact of the technology is limited due to the fact that only one polymer chain end can be functionalized by using the modifier compound described. Accordingly, there is a need for an efficient modification of the second polymer chain end.
EP 2 518 104 discloses a rubber composition for tire application comprising a diene monomer based polymer and silica. The diene polymer is prepared with an initiator to produce living polymer chains having two living polymer ends.
There is a need for modification methods and resulting polymers, including modified polymers, imparting an excellent balance of dynamic properties to crosslinked rubber compounds containing silica such as low hysteresis loss and high abrasion resistance, corresponding to a high wet grip, low rolling resistance and high abrasion resistance in tires, and maintain an acceptable or improved processability. Likewise, it is desirable to provide a polymer which exhibits good processability (a) during the manufacturing process of the polymer itself, for example due to low stickiness during the solvent and moisture removal process, and (b) in the course of the further processing steps such as preparation and processing of silica-filled rubber formulations. These needs have been met by the following invention.